Hi,
as a general rule, any "missing" Great House world mentioned post-3020s is deemed to be a secondary system some place, or an alternative name to a world, since the 3025 maps from FASA are deemed to be 'complete'. We don't have inhabited Great House (or major periphery) systems that are not shown on the maps.
The holdings of nobility has never been clearly nailed down, mostly because it can be rewarded at a whim of a ruler, and can be as small, or as large as said ruler wants.
With regards,
Øystein